Liquid crystal displays (LCD's) are widely used as display devices in products such as word processors and personal computers, television sets, monitors, and mobile information terminals because they can be directly connected to an IC circuit, operated at low voltage and low power consumption, and formed into thin devices. The basic structure of the LCD is comprised, for example, of a polarizing plate provided on both sides of a liquid crystal cell.
Incidentally, the polarizing plate only allows light of a fixed direction in the polarizing plane to pass. The LCD thus has the important role of making visible the changes in the orientation of the liquid crystal caused by an electric field. In other words, the performance of the polarizing plate greatly affects the performance of the LCD. A polarizing plate has a structure in which a transparent resin layer is provided on both sides of a polarizing film composed of a stretched polyvinyl alcohol film on which iodine or a dye is adsorbed. As the transparent resin layer, a film of cellulose ester such as triacetyl cellulose is suitable for a protective film on account of small birefringence, and widely used.
In recent years, a crystal liquid display with large size and high image quality has been developed as a monitor in place of CRT. In accordance with the development, requirement for a protective film for a polarizing plate has been severe, and it is required that a polarizing plate protective film have high quality such as excellent transparency, flatness, flexibility, dimensional stability and mechanical strength and no defects due to foreign matter in view of its usage.
As a cellulose ester film manufacturing method, a solution casting method is generally carried out. The method comprises the steps of casting a so-called dope, in which cellulose ester is dissolved in a solvent such as a halogen-containing solvent, on a rotating endless belt as a support to form a film; evaporating a part of the solvent in the film on the support to obtain a dried and solidied film; peeling the resulting film from the support, and further drying the peeled film, whereby a cellulose ester film is obtained.
It is required in this method that a solvent remaining within the film be removed. Therefore, investment cost for fecilities such as a drying line, drying energy, a solvent recovery apparatus and a solvent reproduction apparatus or manufacturing cost is high. It is important to reduce the cost.
As a means to solve the above problem, a cellulose ester optical film manufactured according to a melt casting method is disclosed in Patent Document 1. This provides an optical film which excels in optical stability, physical stability or dimensional stability, however, the optical film is insufficient in compatibility with various additives, which has problems in storage stability (durability) for long period.
In Patent Document 2 is disclosed a polymer or copolymer of an ethylenically unsaturated monomer having an amido bond, which improves compatibility with cellulose ester and exhibits excellent flexibility, non-volatility and nonmobility. However, it has been found that such a polymer is insufficient in non-volatility and nonmobility to a several tens of μ thick optical film and has a serious problem in slitting property described later. In Patent Document 3 is disclosed a cellulose ester film containing a polymer of an ethylenically unsaturated monomer having an ester bond and a retardation adjusting agent. Similarly, the cellulose ester film is insufficient in non-volatility and nonmobility and has a serious problem in slitting property.
Generally, when a cellulose ester optical film is manufactured, the knurling portions on both ends of the film are trimmed off, the portions at which the film is gripped during stretching in the transverse direction are removed, and the wide web film is slitted to a predetermined width. However, it has been found that methods disclosed in Patent Documents 1 through 3 produce the fault that the section of the film after slitting is roughend and cracked, resulting in lowering of productivity. Further, minute waste film produced during film slitting is floated in air in the film manufacturing process and adheres to the film surface, resulting in faults. Improvement is required.
When a polarizing film protective film is adhered to a polarizing film to obtain a polarizing plate, a cellulose ester film is immersed in an alkaline solution with high concentration at high temperature, i.e., subjected to saponification treatment for surface hydrophilization so as to easily coat an adhesive on the surface, and then the film surface is coated with the adhesive and the resulting film is adhered to a polarizing film through the adhesive. However, it has found that the methods disclosed in Patent Documents 1 through 3 have problems in durability of film after saponification, resulting in abstraction to continuous production of polarizing plates.
Generally, a cellulose ester optical film provided with a functional layer such as a hard coat layer, an antireflection layer or an antistatic layer is provided on the outermost surface of an LCD. However, it has been found that the methods disclosed in Patent Documents 1 through 3 lower layer strength of the cellulose ester optical film provided with the functional layer as above.    Patent Document 1: Japanese Patent O.P.I. Publication No. 2000-352620    Patent Document 2: Japanese Patent O.P.I. Publication No. 2000-212224    Patent Document 3: Japanese Patent O.P.I. Publication No. 2007-119737